Heat assisted magnetic recording (HAMR) utilizes thermal energy to lower the energy barrier for magnetization reversal of magnetic grains in a magnetic storage media. To realize thermally assisted reversal of the direction of magnetization of the grains, the writing temperature should be very close to the Curie temperature (Tc) of the materials used in the storage media. Moreover, Tc should be lower than the boiling temperature (e.g., 650 K) of a lubricant that coats the top of the media.
There is a strong relationship between Tc and magnetic anisotropy (Ku) of magnetic materials. Generally speaking, high Ku materials have higher Tc's. For example, in a single crystalline film of magnetic material such as FeNiPt, when Tc is reduced, the Ku is also reduced significantly. Such physical realities make it difficult to use HAMR efficiently to increase areal density of magnetic recording.
Data bits stored on a storage media are represented by the direction of magnetization of a plurality of grains of magnetic material in the storage media. The direction of magnetization of the grains is set by a magnetic field produced by a recording head in a writing operation. Even when the media is heated to the Curie temperature during writing, the Zeeman energy provided by the interaction of a magnetic field from a recording head and media grain magnetization is very small because the magnetization of the media grains is close to zero. As a result, media grains do not experience a strong difference between the desired writing direction of the magnetization and the opposite direction. Thus, when the recorded bit is cooled, many grains may be frozen into unwanted magnetization directions.
One way to address this issue is to use a data storage media having a FePt/FeRh bilayer structure. When heated, the FeRh layer's magnetization can be instantaneously changed from zero (i.e., the antiferromagnetic state) to above 1000 emu/cm3. If FePt grains are positioned on top of the FeRh grains, the magnetically soft FeRh will reduce the switching field of the FePt grains. However, since both FeRh and FePt are chemically ordered structures, which require thermal processing during manufacture, it is difficult to make either of the thin films into a layer of fine grains. Furthermore, in a previous two layer design, the top layer was used to improve the thermal stability, and the discontinuous bottom layer had a relatively large unit size, which limited the thermal assisted switching effect.
There is a need for storage media that uses high Ku materials that can be switched in a HAMR data storage system.